As Mr. McIlmoyle says, if you weren't charged, you aren't convicted, and therefore no airsoft weapons ban is in effect. As such, there is no need for a 'pardon'. That said, you are definitely now in the police system as a subject contact regarding a weapons call.
The difference between Officer Friendly giving you a lecture, and a tactical unit force-testing your apartment locks in a take-down probably came down to the color of the airsoft gun that you were irresponsibly mishandling. Whomever called the police, at least knew that a clear weapon was likely non-lethal, and reported it as such, and the police responded appropriately.
If you so choose to take part in this sport, do so ultra-responsibly. I'd suggest a guitar case for storage, and never take it out of that case outside of a playing field, unless it's in an interior room with no windows. Otherwise, if previously aforementioned neighbor/observer decides to make a repeat call with an updated weapons description, Officer Friendly and his backup team won't be quite so quick to deliver just stern advice first..
I've sort of been down that road before.. and in my own opinion, not really any individuals involved were at fault.. but it did result in a full-on takedown of my college shared residence - a dog catcher picking up a mutt, reported a spitball airgun with a wood-stock carelessly stored on a coffee table as a sawed-off shotgun, and the police using the pretext of a minor police call a few weeks before, knocked out all stops in the repeat visit. Even 20+ years on, that was an experience that i'd personally not like to repeat again in my lifetime - and i wasn't even in the residence at the time.